This invention relates to an electronic distance measuring device of the kind in accordance with the preamble of claim 1.
Electronic distance meters (EDM) which measure by directing an electromagnetic beam, such as a light beam, towards a target and collecting reflected radiation are principally of the phase measuring or the propagation delay time measuring kind. Some of these kinds of measuring instruments are adapted for making distance measurements against natural objects, i.e. against objects which are not provided with reflectors, for example prism reflectors, such as cube corner reflectors. A phase measuring distance meter uses a more or less continuous emitted beam, i.e. a beam provided with an almost symmetric modulation with one or more frequencies, but there are also phase measuring instruments working with short pulses. A propagation time measuring distance meter uses short pulses, which are emitted repeatedly with a determined pulse emission rate where the time interval between pulses is longer than the expected time for a pulse to propagate to and from the target.
Typically, the propagation time measuring system has a longer range than the phase measuring system since the intensity of each pulse can be higher. The most common light sources are pulsed laser diodes.
A number of difficulties and disadvantages with the prior known distance meter constructions for measuring with diffuse reflexion against natural targets can be summarised in the following way:
The light to be used often has a wavelength that lies outside the visible wavelength band. This leads to the fact that the operator does not see the target point against which the measurement is taken.
The emitting area of the light source must have a certain width to be able to provide a power that is high enough for a reasonable range. This leads to a beam with a rather wide divergence and that the measuring point, i.e. the lightened are, will have a large diameter when the distance to the target is long.
The emitted cone angle from the light source is wide. This leads to a large diameter of the optical system of the transmitter if there is a wish to emit a narrow beam (a long focal length) and at the same time have an output power which is large enough to get a reliable measuring result. This leads in turn to a wide beam diameter at the target when measurements are made to short distance targets.
The light transmitter and receiver use different optical system units or different parts of the same optical system with some kind of separating wall between them. When the transmitter and receiver optical system are adjusted to be parallel to each other in order to provide a maximum signal amplitude when measuring towards distant targets, then the receiver will not xe2x80x9cseexe2x80x9d the lightened area when measurements are made towards nearby targets. This means that a reflected signal will not be obtained when the measurement is made towards nearby targets positioned at a distance shorter than a certain distance from the distance meter.
EP 706 063 (COMMISARIAT A L""ENERGIE ATOMIQUE) describes a microchip laser, which is passively Q-switched, for use in a distance meter. The advantage is that this kind of laser generates a narrow, pulsed beam having a minor divergence. However, the passive Q-switching gives a problem due to the fact that the exact time for emitting the light pulses can not be controlled. A solution of the problem for measuring towards nearby targets is not mentioned or discussed. The electronic solution assumes detection of separate pulses. This limits the range since it presupposes that every received pulse has an amplitude which is higher than the background noise. Since, the present invention relates to an electronic distance measuring system using a microchip laser EP 706 063 has been used as the basis for it and is hence disclosed in the preamble of claim 1.
PCT/SE97/00396 (GEOTRONICS AB, which now has amended its name to SPECTRA PRECISION) describes a system in which received pulses could have a lower amplitude than the background noise. This system demands that the emission time instant of the emitted pulse is controllable, or at least well defined and without jitter. The present invention has been developed in order to provide a well functional distance measuring system based on for example this application. However, the present invention is not restricted to use exactly the kind of microchip laser disclosed in PCT/SE97/00396.
Some efforts have also been made to eliminate some of the disadvantages described above, however for a phase measuring system. EP 701 702 (Leica AB) describes a system in which a laser diode is used having a wavelength within the visible wavelength band. This is accomplished with the requirement that a phase measuring distance meter must be used, since laser diodes emitting within the visible wavelength band are not able to provide pulse power high enough for propagation time measurements within reasonable distances, i.e. exceeding 50 m. It is mentioned that the pulse length could be as low as 2 ns, but nevertheless it is a phase measuring system. This system allows, however, the use of a well collimated light beam but the range is too short for most applications.
An object with the invention is to provide an electronic distance measuring system, which solves all of the disadvantages stated above.
A particular object of the invention is to provide a well functioning distance measuring system comprising a microchip laser as the radiation source.
One object of the invention is thus to provide an electronic distance measuring system with a narrow transmitted beam in order to establish a well defined measuring spot on the target able to measure towards nearby targets as well as to distant targets.
Still another object of the invention is to provide an electronic distance measuring system able to make long range measurements, for example to targets more than 50 m away.
Another object of the invention is to provide an electronic distance measuring system in which the point on the target towards a measuring is to be made is visible for the operator of the system, at least during adjustment of the sighting towards the target.
The objects stated above are solved with an EDM system having the features disclosed in the independent claim. Further features and further developments are disclosed in the rest of the claims.
The invention has been developed to use a microchip laser, where the instant for emitting a beam pulse is controllable, in a distance measuring device. This kind of possible light source could be an actively. Q-switched laser or a combined actively and passively Q-switched laser of the kind disclosed in the Swedish patent application No 9702175-2 (SPECTRA PRECISION).
The light source disclosed in the specification mentioned above generates an emitted beam having a small diameter. By means of a beam expander the beam divergence can be minimised to a suitable level and the beam can be given an adapted beam diameter. For example, it is possible to choose a divergence of 0.3 mrad and to have a beam diameter of less than 5 mm. The microchip laser emits normally light within the infrared (IR) wavelength band. However, a frequency doubling crystal could be provided at the output of the laser or inside the laser cavity in order to generate visible light.
The invention relates to an electronic distance measuring device which measures the propagation time to and from a target of a short pulse of a transmitted electromagnetic beam from a radiation source comprising a microchip laser, and comprising objective optical system for the reflected beam from the target and a detecting unit to which the received beam is transmitted. Means is provided for making the transmitted and the receiving electromagnetic beam coaxial. Means are also provided for focusing the received beam onto the detecting unit. A means makes the transmitted electromagnetic beam simultaneously as narrow and as collimated as possible in order to get a well-defined radiation measuring spot on the target. A control means determines the times of each transmitted electromagnetic pulse by means of a fixed electronic reference.
The device preferably comprises means for making the transmitted and the received electromagnetic beams coaxial, and target adaptation means for enabling the optical system to direct part of the received beam reflected from the target to the detector means irrespective of whether the target is a nearby or distant target. The target adaptation means could comprise a flexible optical fibre coupled to a detector in the detector unit. The free fibre end could be moved manually or by a actuator, such as a motor, to a position in which the measuring point on the target is focused.
Alternatively the target adaptation means could be a lens means movable along the optical axis of the receiving optical system and providing different focusing for different positions, the target adaptation means being moved manually or automatically to a position where the reflected beam from the target is focused on the detector unit. Another alternative is to let the target adaptation means be an object lens system in which parts of the aperture have different focal lengths such that both the shortest and the longest target limits to which measurements could be made are focused to the detector unit by some part of the aperture. For instance, a ring-shaped lens means covering a pail of the incoming beam reflected from the target and in co-operation with the objective optical system could then focus beams coming from the minimum range for a nearby target on the detector unit while the optical system not covered by the ring-formed lens could focus beams coming from distant targets on the detector unit.
It is possible to let the radiation source emit light within the visible wavelength band. However, it is also possible to let the radiation source means emit visible light during an adjustment period for aiming the device towards the target before the actual measurements, and to let the radiation source means emit electromagnetic beam pulses in a wavelength outside the visible wavelength band during the actual measurements. It is also possible to use two different sources, one visible for pointing and one IR source for measurement. It is possible to have these sources emitting radiation simultaneously.